1. Field of the Invention
This invention relates to a luminance control apparatus suitable for a personal computer using as a display unit a flat panel-type display unit such as a liquid crystal display (LCD) panel or a plasma display panel (PDP), and more particularly, to a battery driven portable personal computer.
2. Description of the Related Art
In the field of personal computers, small-sized, light-weighted, and portable lap-top type computers have become widespread, replacing the conventional desk-top type computers. Such personal computers use flat panel type displays such as LCD and PDP. Most of these computers are powered by batteries.
For the flat panel display units used in the portable computers power dissipation changes depending on the display luminance. A method of controlling the luminance of the display device by directly or indirectly detecting the dissipated power or the heat produced by the power dissipation has been proposed by the same assignee as that of the present invention and is disclosed in Laid-Open Japanese Application No. 60-216446.
Generally, a display is performed by converting energy into a form of luminance. Thus, the power consumption is changed according to the luminance level, and heat is produced accordingly. The PDP is a display unit in which electrical discharge is directly viewed. For this reason, as the discharge energy, i.e., the dissipated power increases, the display luminance increases. The LCD is a display unit in which the liquid crystal serves as a shutter shielding the light generated by the backlight. Increased power of the backlight increases the luminance. Reflective type LCD is an exception in which the luminance is not directly related to the power consumption since reflection of the external light is used.
To restrain the heat production, the display luminance is controlled by the detection of power consumption, as discussed in the abovereferenced Japanese Application No. 60-21446. For example, a DC type PDP is known which has a built-in type circuit for controlling the luminance to prevent a current of more than a certain quantity from flowing.
For the purpose of reducing heat, the existing circuit satisfactorily works, but in a battery-driven lap-top type personal computer, power consumption must be restrained not only for reducing heat dissipation but also for the purpose of prolonging the battery operation time.
However, as in the DC type PDP, a display unit incorporating the existing circuit for keeping the power dissipation under a certain level or less, cannot detect a condition under which a battery is used and the change of the residual capacity of the battery. For this reason, the power consumption and the display luminance are constant when the battery is not used with an AC adapter connected, or when the battery is used and the residual capacity is small. When an AC adapter is connected, the power dissipation of the battery need not be considered. For this reason, the maximum luminance could be obtained while limiting the heat to a certain level. In the DC type PDP, the power consumption is increased also by the size of display on the screen. However, during the use of battery with small residual capacity, if the power consumption is abruptly increased, such as when a reverse display is abruptly made, battery current abruptly stops flowing. For this reason, the battery power is cutoff and the shutdown operation due to the battery cutoff may be performed (in the fail state). Thus, there is a drawback that it is necessary to perform the shutdown operation earlier than necessary and thus the operation time is shortened.
As mentioned above, in a battery-driven type portable personal computer, the power consumption must be reduced to prolong the battery operation time. For this reason, in the display unit in which power consumption varies according to the display luminance, the display luminance must be controlled to be constrained to be lower than the conventional luminance control for constraining the heat, from a view point of detecting the power dissipation and controlling the display luminance.
As shown in Japanese Utility Model Application No. 59-147469 which has been filed by the same assignee, a system using a DC type PDP, is known having a micro computer in the conventional power supply device, for detecting the use and residual capacity of a battery in addition to the normal control such as control of the output voltage.